Estrogen receptor (ER)-targeting endocrine therapies that interfere with estrogen signaling have become a standard treatment for breast cancer. These treatments involve either direct ER binders (e.g. tamoxifen) or aromatase inhibitors that reduce mammary tissue estrogen levels (e.g. letrozole). Despite being an effective treatment for ER-expressing breast cancer, current therapies have significant limitations related to inherent or acquired resistance. Our recent work has characterized a DNA-intercalating agent (designated XR5944) which possess anticancer properties through a novel mechanism of action;inhibition of gene transcription via direct binding at critical DNA regulatory sites. We have demonstrated potent antiestrogenic activity of XR5944 through specific inhibition of the binding of ER with estrogen response elements (ERE). As a result of these studies, we hypothesis that small molecules which specifically target ERE can be promising drug candidates for the treatment of breast cancer and would be effective in overcoming the resistance to current antiestrogen therapies. Using XR5944 as a prototype, it is our overall goal to develop such compounds using state-of-the- art molecular and in silico techniques. Aim 1 will characterize the nature and specificity of ERE as the molecular target of XR5944 action in breast cancer cells. As part of this aim, we will perform a genome-wide identification of the DNA-binding sites for ER which are affected by XR5944 and, in so doing, determine the target genes and downstream molecular pathways that are affected by XR5944-DNA interactions. Using nuclear magnetic resonance (NMR) technology, aim 2 will determine the detailed structure of the XR5944-ERE complex, and use this information, along with computer molecular modeling studies, as a basis for designing better ERE inhibitors. Upon identification of promising new compounds, they will be synthesized and promising candidates further tested for biological activity and mechanisms of action. Structural features that promote ERE binding and subsequent ER inhibition will be utilized to guide further drug improvement. Finally, aim 3 will evaluate the specificity and therapeutic consequences of blocking estrogen signaling by the ERE- intercalating compounds in two in vivo mouse models;an aromatase transgenic model which gives rise to estrogen-dependent breast tumors, and a wild-type Sencar (SENsitivity to CARcinogenesis) mouse model with which to determine the specificity and effects of the compounds on estrogen-independent cancers. PUBLIC HEALTH RELEVANCE: Breast cancer is a major public health issue that will affect approximately one of nine women sometime during her life. Since current endocrine therapies for ER-positive tumors have limitations, new treatment modalities are desperately needed. In addition to providing a basic framework for the development of new antiestrogen compounds, these studies will establish a new concept of inhibiting DNA/transcription factor interactions with small molecule DNA binders.